Image: Richard Ditch, 2008 [larger view].
Please name at least one field mark that supports your identification.
Rick Wright, Managing Director of WINGS Birding Tours Worldwide, writes:
There’s no question that this is a small bird, is there? If we start at the back, we find a shortish, narrow tail stuck on to a comically pudgy body; the shape and the extensive yellow-green edging on the tail feathers that immediately rule out a vast number of small passerines, including all the parulids. The wing feathers show the same edging, along with two clear, broad wingbars, the upper one mostly concealed by feathers overlapping it from the bird’s back. This photo has many merits, among them the way that it shows just how such gross marks as wingbars are created by the fine details of feather patterns: wingbars are formed by contrastingly colored tips to the secondary coverts, the lower bar by the greater coverts, the upper bar by the median coverts.
In a view as splendid as this one, we can analyze the wing pattern in further and productive detail. The tertials are blackish with bold white edges, the secondaries extensively yellow and the primaries heavily edged grayish-white. The “minor” feathers of the wing are strikingly blackish: the alula (beautifully visible in this image) and the greater coverts. Only a few of the median coverts are visible, and even on those only the tips. The lesser coverts, as so often on passerines, are entirely concealed.
This wing pattern actually allows us to identify the bird without going further. Among the small birds of North America, only Hutton’s Vireo shows two equally well-defined white wingbars bordering the blackish rectangle formed by the greater coverts. On Ruby-crowned Kinglet, otherwise remarkably similar in plumage, the greater coverts are much paler, hardly contrasting with the folded flight feathers, and the lower wingbar is considerably better developed than the upper. The blackest part of the kinglet’s wing is below the lower wingbar, a black patch formed by the contrasting base of the secondaries; on the vireo the blackest part is above that wingbar.
There are some important structural differences, too, that will permit the two species to be distinguished in views worse than this. Notice how fat-headed and neckless this bird is; kinglets are a bit more finely built, their heads obviously narrower than their shoulders. And the bill of a kinglet is much sharper and finer than the absurdly blobby bill protruding from the vireo’s face.
Another useful mark is just barely visible in this photo. Look at the toes of this bird: they are moderately thick, as would be the tarsus if we could see it. The toes and tarsus of a Ruby-crowned Kinglet are oddly slender, the result of the different scaling pattern of the two species’ feet: the vireo’s tarsus is scutellate, covered with the usual overlapping scales; the kinglet’s, though, is booted, with a single long scale extending from the ankle almost to the top of the toes. This difference is also responsible for the pale, translucent color of a kinglet’s foot; the thinness of the tarsus and its translucence are often the most easily seen characters on a tiny greenish bird fluttering high in a tree.
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